Abstract
Under the natural conditions the silk gland cells of the silkworm of yellow cocoon type reared on mulberry leaves were found to absorb carotenoids from the blood during the last larval instar to spin yellow cocoon. However, if an artificial semisynthetic diet containing no carotenoid was given to the larvae of the last instar, they produced white cocoon even if their genotype was composed of a set of genes for pigmentation.On the other hand, if β-carotene was added into the semisynthetic diet only for the last larval instar, the cocoon colour and the order of pigmentation in each region of silk gland of the yellow cocoon type were completely the same as those of the European fleshcoloured cocoon type. These results suggest that the expression of the pigmentation regulated genetically is restricted by nutritional conditions during the last instar. Moreover, it seems that the initiation of permeation of pigments at the middle division of silk gland depended on the development of the silk gland even if the temperature was varied (24.5±1.0°C, or 18.0±0.5°C) in so far as larvae were reared on mulberry leaves, but it did not depend on that of the silk gland in case that larvae were fed on different artificial semisynthetic diets.
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